mine does the same... usually starting at 4 and turning it down works...
there are resistors for the slower speeds - since it works sometimes, they're obviously still OK but connections might be getting flaky - if you need the wiring diagram for the fan, send me an email and I'll scan it for you.
Wilf - Toronto, Ontario.
'99 E36 M3 Convertible
'88 E30 325i Convertible